Differentiation and emplacement of the Worthington Offset Dike of the Sudbury impact structure, Ontario.

Abstract

The Offset Dikes of the 1.85 Ga Sudbury Igneous Complex (SIC) constitute a key topic in understanding the chemical evolution of the impact melt, its mineralization, and the interplay between melt migration and impact-induced deformation. The origin of the melt rocks in Offset Dikes as well as mode and timing of their emplacement are still a matter of debate. Like many other offset dikes, the Worthington is composed of an early emplaced texturally rather homogeneous quartz-diorite (QD) phase at the dike margin, and an inclusion- and sulfide-rich quartz-diorite (IQD) phase emplaced later and mostly in the centre of the dike. The chemical heterogeneity within and between QD and IQD is mainly attributed to variable assimilation of host rocks at the base of the SIC, prior to emplacement of the melt into the dike. Petrological data suggest that the parental magma of the Worthington Dike mainly developed during the pre-liquidus temperature interval of the thermal evolution of the impact melt sheet (>1200 °C). Based on thermal models of the cooling history of the SIC, the two-stage emplacement of the Worthington Dike occurred likely thousands to about ten thousand years after impact. Structural analysis indicates that an alignment of minerals and host rock fragments within the Worthington Dike was caused by ductile deformation under greenschist-facies metamorphic conditions rather than flow during melt emplacement. It is concluded that the Worthington Offset Dike resulted from crater floor fracturing, possibly driven by late-stage isostatic readjustment of crust underlying the impact structure.